Recently, in order to aim to improve fuel efficiency of an automobile, a reduction in a drive loss of an automatic transmission is required. A seal ring is mounted in the automatic transmission for the purpose of hydraulic sealing, but a friction loss of the seal ring leads to a drive loss of the automatic transmission. For this reason, a reduction in friction of the seal ring becomes an important problem. Further, because a capacity of an oil pump of the automatic transmission accounts for a great part of the drive loss, a reduction in an amount of oil leakage from the seal ring and a reduction in a capacity of the oil pump are required. As such, in order to reduce the drive loss of the automatic transmission and improve fuel efficiency of the automobile, a low friction function and a low leakage function are required for the seal ring.
FIG. 1 illustrates a basic structure of a hydraulic circuit using the seal ring. A seal ring 1 is mounted in a shaft groove (ring groove) 4 that is formed at both sides in an axial direction of a hydraulic pressure passage 3 of an outer circumferential surface of a shaft 2. A pressure receiving side surface 11 and an inner circumferential surface 12 of the seal ring receive operating oil supplied from the hydraulic pressure passage 3, an outer circumferential surface 13 of the seal ring comes into contact with an inner surface of a housing 5, and a contact side surface 14 of the seal ring comes into contact with a side surface of the shaft groove 4, thereby sealing the hydraulic pressure. In general, the shaft 2 rotates and the housing 5 is fixed, but there is also a combination reverse to the former combination.
In order to reduce friction (friction loss) of the seal ring, a method is usually adopted which reduces a pressure receiving load that presses the contact side surface of the seal ring, which becomes a main sliding surface, against the ring groove. Specifically, a seal ring is adopted which has a cross-sectional shape in which the supplied hydraulic pressure is applied between the contact side surface of the seal ring and the ring groove, thereby reducing the pressure receiving load by an operation of a cancellation load.
Patent Literature 1 discloses a method in which a side surface of a seal ring is formed in a tapered shape in which a width in an axial direction is decreased from an outer circumferential side to an inner circumferential side, so as to generate a cancellation load between the side surface of the seal ring and a ring groove, thereby aiming to reduce a pressure receiving load. The tapered shape of the side surface may drastically reduce the pressure receiving load, and has been known at present as a shape of the seal ring, which causes the smallest friction.
Further, as illustrated in FIG. 2(A), Patent Literature 2 discloses a seal ring having concave portions (pockets) 6 formed to be spaced apart from each other in a circumferential direction at an inner circumferential side of at least a contact side surface, and a pillar portion 7 disposed between the concave portions 6. As illustrated in FIGS. 2(B) and 2(C), the concave portion 6 is formed by a deepest inclined portion 51 provided so that a width (thickness) in an axial direction of the seal ring becomes thin in an inner circumferential direction, and converging portions 52 positioned at both sides in a circumferential direction of the deepest inclined portion 51 and of which each converges to a point that is the closest to the inner circumferential side of the adjacent pillar portion 7. In this configuration, the friction is reduced by a lifting force 60 which is generated by squeezing oil filled in the concave portion 6 on an inclined surface of the converging portion 52 by rotation of the seal ring, and an effect (cancellation pressure 61) of reducing a pressing load by applying hydraulic pressure in the concave portion 6 of the contact side surface. In addition, as illustrated in FIG. 2(D), in the seal ring of Patent Literature 2, because the side surface of the seal ring comes into sliding contact with the ring groove with surface contact, a leakage flow path of an abutting end gap is not formed, and low leakage characteristics are obtained.
Moreover, Patent Literature 3 discloses a seal ring in which a groove, which is opened in an inner circumferential surface, broadened in an outer diameter direction and a circumferential direction from a deepest portion at the inner circumferential surface side while going forward in a rotational direction of a rotating shaft, and becomes gradually shallow in the outer diameter direction and the circumferential direction from the deepest portion, is provided in a side surface of the seal ring. It is disclosed that in this configuration, an oil film may be widely formed on a side surface that slides with a side wall surface at an anti-seal fluid side of a ring-shaped groove, abrasion may be reduced by eliminating direct contact with the side wall surface, and durability is excellent.
In the seal ring of Patent Literature 1, there is a problem in that an oil leakage occurs from an abutting end gap because the sliding contact between the side surface of the seal ring and the ring groove is a line contact, and a sliding diameter is positioned on the abutting end gap of the seal ring. Meanwhile, friction is reduced by adopting the concave portion of Patent Literature 2, but because this effect is lower than the effect of the seal ring of Patent Literature 1, a seal ring having a more excellent effect of reducing friction is further required. Further, in the configuration of the oil groove (concave portion) of Patent Literature 3, an occurrence of the lifting force by squeezing oil may not be expected, and there is a limitation to reduce friction.